1. Field of the Invention
The present invention relates to a laser interferometer, a position measuring apparatus, and an exposure apparatus, and more particularly to a laser interferometer and a position measuring apparatus for superposing a measurement beam reflected from a moving mirror and a reference beam reflected from a reference mirror, and measuring the position of a moving object, and to an exposure apparatus in which this interferometer or position measuring apparatus is provided as a position measuring apparatus for at least one of a mask stage and a substrate stage. The present invention also relates to a position measuring method and to a method for manufacturing a laser interferometer, a position measuring apparatus, and an exposure apparatus. The laser interferometer and position measuring apparatus pertaining to the present invention are particularly suitable for use as scanning exposure apparatus in the manufacture of liquid-crystal display panels, plasma display panels, and other display elements.
2. Description of the Related Art
Scanning exposure apparatus capable of exposing large surface areas in a single sweep are currently used on a comparatively large scale to accommodate increased dimensions of liquid-crystal substrates used in lithographic processes for manufacturing liquid-crystal display panels or the like. As disclosed, for example, in U.S. Pat. No. 5,729,331 corresponding to Japanese Patent laid-open No. 07-57986, known scanning exposure apparatus for such liquid crystal applications are configured such that a projection optical system is constructed using a plurality of same-size, erect projection optical units composed of two partial optical systems each of which contains a refraction system and a concave reflecting mirror, and a mask and a plate (substrate) are moved as a unitary structure relative to the projection optical system to achieve single-scan exposure.
An increase in the size of a liquid-crystal substrate makes it necessary to increase the scanning distance for the mask stages and substrate stages in a scanning exposure apparatus used in liquid crystal applications. Consequently, reflecting mirrors in which the length of the reflecting surfaces in the scanning direction exceeds the length of the stages in the scanning direction must be provided as moving mirrors or fixed mirrors in order to achieve high accuracy when measuring the positions of mask and substrate stages in a non-scanning direction by a laser interferometer. A scanning exposure apparatus equipped with a fixed mirror that extends a considerable distance in the scanning direction is disclosed, for example, in Japanese Patent Laid-open No. 10-74692. Disclosed in this publication is a double-pass laser interferometer for measuring the position of a stage in a non-scanning direction, wherein a beam splitter (for dividing laser light into a reference beam and a measurement beam), a mirror, a corner cube, and the like are fixed to a stage, and a fixed mirror consisting of a plane mirror that is longer than the stage is used as a reference mirror.
Recently, however, liquid-crystal substrates have grown in size even further, gradually making it more difficult to use the technique described in Japanese Patent Laid-open No. 10-74692 without further modifications and to measure the position of a stage in a non-scanning direction with high accuracy. This is attributed to the fact that stages have recently grown in size to match the increased size of substrates, so the aforementioned fixed mirrors have become longer, making it more difficult to process and polish reflecting surfaces thereof with sufficient accuracy.
Liquid-crystal substrates and the like will undoubtedly continue to increase in size in the future, and an urgent need therefore exists to develop a novel technique capable of accommodating this trend.